Ya sure you can use t-mobile sim in your phone only after unlocking it from AT&T network provider .You can easily unlock it using sites like ClassicUnlocking.com here they are rendering network unlock code for all mobile models at reliable cost .You first check that whether the code is available for your mobile model ,country and network provider .If yes then pay for it and unlock your phone then freely use it with t-mobile or any other GSM sim ....